Should I put a cap on the waste drain when removing a dishwasher

most of these wastes have a trap on them,if it has a trap there is no need to cap it,if not then cap it.

Hello, no question is stupid if you don’t know the answer. Yes you do need to cap the outlet that’s no longer required. Ideally with the original plug it came with (normally blue in colour) or removing the dish washer tail and fitting a cap in its place. If you don’t have these the next easiest option is to fit a new single (one outlet) sink waste (the fitting that screws onto your sink bowl.) These are cheap and widely available, and will fit into place with no fuss.

Pipe End Caps are a quick, simple, cost effective and reliable product used to provide temporary water and dirt seal at the end of a plastic or metallic pipe. ... Caps are available to suit virtually all sizes of plastic or metallic pipe from 16mm up to 1600mm. I'm sure you will be able to find the right one at a screwfix store

Just cover it up with a bit of gaffer tape

Yes, would stop any smells if theres no water in the trap and it also saves from any debris going down it , depending on the size somewhere like screwfix does everything you need !